In the conventional power conversion system, AC-DC converters are used to convert the AC input voltage VAC supplied by the grid power to a desired DC voltage VDC which is then modulated by the voltage converter generating a final DC output voltage VOUT with small ripples. The conventional technique employed in the AC-DC conversion system of prior art faces a problem is that the peak or effective value of the AC input voltage VAC is not constant at all times. When AC input voltage VAC supplied from the grid power is in brown-out or overvoltage condition, it may cause damage to the AC-DC converter. Thus real-time monitoring and determination of the trend of AC input voltage VAC is essential.
In the U.S. Patent Publication US20090141523, two resistors in series are used to form a voltage divider and a detection voltage reflecting the changes in the input voltage VAC is produced at a common node between them. The two resistors are connected in series between the output terminal, at which the voltage is VM, and the ground. It is well known in the art that the current flowing through the two resistors, as a result, the resistors consume power, even though the two resistors merely serve as secondary detection components. In view of this, the requirement of a device that can effectively detect changes in input voltage VAC, thus accurately reflects the changing trends of voltage VAC, but at the same time able to avoid unnecessary excessive power consumption is a big challenge.
It is within this context that embodiments of the present invention arise.